1. Field of the Invention
The present invention relates to a semiconductor laser device and, more particularly, to a semiconductor laser device with a multiquantum barrier.
2. Description of the Related Art
Recently, semiconductor laser devices with double hetero-(DH) structures, capable of emitting red light having wavelengths of from 650 to 670 nm with high efficiencies, have been developed. Semiconductor laser devices of this type have advantages such as a low threshold current density, a high maximum operating temperature, and a long operating life.
If, however, the DH structure is applied to a semiconductor laser device having an oscillation wavelength of less than 650 nm, problems such as a high threshold current density and a low maximum operating temperature arise. Short-wavelength laser devices are required to realize high-density recording in optical memories and are also expected to be used as the light sources for bar-code readers. A semiconductor laser device would be expected to an attractive alternative light source for bar-code readers. This is because standard bar-code readers currently being used employ a helium-neon gas laser for the light source, having a wavelength of about 630 nm, and also the use of a semiconductor laser device enables considerable miniaturization of the light source.
It is assumed that the increase in threshold current density and the decrease in maximum operating temperature observed in currently used semiconductor laser devices, with a DH structure, operating at short wavelengths lengths of less than 650 nm are mainly due to the following reason.
That is, as the oscillation wavelength is shortened, the height of the hetero barrier in the conduction band between the active layer and the p-type cladding layer is decreased, and this allows carriers to flow from the active layer into the p-type cladding layer, thereby constituting a carrier loss.
In comparison to the aforementioned laser structures when an active region with an MQW structure is employed, its density of states distribution function exhibits a steplike characteristic, and the energy distribution of the electrons is highly localized, causing an increase in the laser gain, and thus a lowering in the threshold current density. Since the energy distribution of the electrons varies only slightly with temperature, the temperature characteristics of the device is improved, in other words, the maximum operation temperature is increased.
Thus, a semiconductor laser device having a double hetero-(DH) structure including an active layer with an MQW structure exhibits a low threshold current density, a high maximum operation temperature, and a long life, as compared to those of the semiconductor laser devices having a double hetero-structure other than the above.
However, a semiconductor laser device having an oscillation wavelength of less than 650 nm, involves such drawbacks as high threshold current density and a low maximum operation temperature. Therefore, it has been difficult to meet the recent demand for increasing the recording density, by shortening the laser wavelength used, of the optical memory devices.
The drawbacks regarding the high threshold current density and the low maximum operation temperature are due to the phenomenon whereby electrons in the active layer leak more and more easily into the p-type cladding layer as the oscillation wavelength is shortened. Such a leak of electrons can be further decreased by use of an active layer which contains strain. However, even with this method, the leak of electrons cannot be reduced to a satisfactory level for solving the above-described drawbacks.
To solve this problem, methods of forming a multiquantum barrier (MQB) between the active layer and the cladding layer has been proposed (Kishino et. al., Appl. Phys. Lett. 58 1991, p. 1822; Iga et. al., Electron Lett., 22 1986, p. 1008).
There was proposed a p-type cladding layer having a multiquantum barrier structure. The p-type cladding layer having an MQB structure will have a higher effective electron barrier against electrons in the active layer as compared to the other types of p-type cladding layers, having no MQB structure. This is because the MQB structure serves as an extra barrier produced in terms of the quantum confinement effect in the structure.
Design and fabrication of laser devices having an MQB structure (MQB semiconductor laser devices), however, involves overcoming the following problem.
That is, the MQB structure involves a large number of parameters, such as the quantum well width and the associated barrier width. To further complicate matters these parameters are mutually associated with each other. Therefore, it is difficult to set all these parameters optimally, and this in turn makes it difficult to fabricate a highly optimized MQB semiconductor laser device.
As described above, since a large number of parameters are related to the laser characteristics of conventional MQB semiconductor laser devices, these devices are difficult to design and are therefore poor in practicability.
By employing the MQB structure, laser devices which have been able to suppress the leak of electrons from the active layer into the p-type cladding layer have been developed; however, the advantage obtained by the MQB structure is not as good as expected. That is, the increase in the height of the barrier, which was theoretically expected, could not be achieved, and therefore the leakage of the electrons could not be suppressed to a low enough level sufficient for solving the above problem.
As described above, in the case of a short oscillation wavelength less than 650 nm, the problem of leaking of electrons from the active layer exhibits a prominent effect, causing a high threshold current density and a low maximum operation temperature.
In order to solve the above problem, there were proposed some techniques in which a strained active layer and a p-type cladding layer having an MQB structure were used. However, with these techniques, it was still not possible to reduce the leakage of electrons to a satisfactory low level, and the above problem remains unsolved.